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Progressive Addition Lenses
The concept of progressive addition lens has been around since 1907 when the first patent on progressive power lens was published by Owen Ave. The early progressive lenses were rather crude in design. Varilux 1 was introduced by Essilor in France in the year 1959 and since then progressive addition lens have gained worldwide acceptance as the most performing ophthalmic lens for the correction of presbyopia because they provide comfortable vision at all distances. They are successfully and advantageously replacing single vision and bifocal lenses. Growing popularity of progressive addition lenses has stimulated the search for advances in design and manufacturing technology of the progressive lenses. This has enabled progressive addition lenses to develop from the early “hard” designs requiring extended patient adaptation time to current “state of the art” softer asymmetric design to the individually customized progressive lenses. Today over 150 progressive addition lens designs are in the market.
Progressive addition lenses (Fig. 11.1) are one piece lenses that vary gradually in surface curvature from a minimum value in the upper distance
Fig. 11.1: Progressive addition lens
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portion to a maximum value in the lower near portion. Unlike bifocal or trifocal lenses, progressive lenses ensure that the presbyopic spectacle wearer finds the right dioptric power for every distance, guaranteeing smooth and uninterrupted vision without any visible line of demarcation. The power increase is achieved by constantly decreasing the radii of curvature in the vertical and horizontal directions. Figure 11.2 demonstrates the gradual increase in curvature and surface power towards the lower, near portion of the lens.
Fig. 11.2: Cross-sectional view of progressive addition lens surface. The shorter radius of curvature in near portion provides a stronger surface power than the longer radius of curvature in the distance portion
In the most used zones of vision, virtually aberration-free vision is possible, as here the radii of curvature are almost identical in the vertical and horizontal directions.
A typical, general-purpose progressive lens will have three district zones of vision as shown in Figure 11.3.
Fig. 11.3: Distance, intermediate and near zones of a typical progressive lens
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Distance
A designated zones located in the upper portion of the lens, which provides the necessary distance correction.
Near
A designated zone in the lower portion of the lens, which provides the necessary near addition or near power.
Intermediate
A corridor in the central portion of the lens connects these two zones, which increases progressively in plus power from the distance to near. This zone is also known as “progressive zone”.
These three zones of vision blend together seamlessly, providing the wearer with a continuous depth of field from near to far.
Fig. 11.4: Typical progressive lens
BASIC DESIGN DIFFERENCE BETWEEN PROGRESSIVE, SINGLE VISION, BIFOCAL AND TRIFOCAL LENSES
A single vision reading lens consists of a single sphere of appropriate radius providing correction for near vision only. Distance vision through the lens is blurred and there is no specific correction for the intermediate vision. (Fig. 11.5).
Fig. 11.5: Basic design principle of single vision lens
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They provide larger field of view for the reading and they are readily available at low cost. But they are inconvenient to use as they correct only one particular distance at a time.
Bifocal, trifocal and progressive lens designs combine areas of correction for both distance and near vision in a single lens and link them in different ways:
In a bifocal lens, a distance vision sphere is placed above a near vision sphere and is linked by a single “step” creating a visible segment line. They offer convenience of distance and near in the same spectacle and are available at lower cost. But they are unappealing cosmetically as there is a visible demarcation between zones. Intermediate is blurred and the subject have prismatic image jump (Fig. 11.6).
Fig. 11.6: Design principle of bifocal lens
In a trifocal lens, a third sphere is added between the distance and near vision sphere to produce an intermediate power. This gives rise to two segment lines on the lens surface (Fig. 11.7).
Fig. 11.7: Trifocal lens design principle
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In a progressive lens, an uninterrupted series of horizontal curves link distance vision zone, intermediate vision zone and near vision with no visible separation. Lens power increases smoothly from distance vision area at the top of the lens, through an intermediate vision area in the middle, to the near vision area at the bottom of the lens. They provide convenience of use with no prismatic image jump. The subject feels confidence in negotiating stairs; gutters etc., besides progressive addition lenses are under constant research and development, providing us a newer and better design to adapt faster (Fig. 11.8).
Fig.11.8: Progressive lens design principle
ADVANTAGES OF PROGRESSIVE ADDITION LENSES
Most eyewear professionals are aware of the product benefits afforded by the optical features of a progressive addition lens:
No Visible Segments
No line of demarcation provides more cosmetically appealing lenses with continuous vision, free from visually distracting borders. The lens looks like a single vision lens (Fig. 11.9).
Continuous Field of Clear Vision
Progressive addition lens offers a greater visual flexibility with uninterrupted clear vision from distance to near. Single vision reading lens offers a field of clear vision limited to the near area only, while the abrupt change of power in a bifocal creates completely divided fields for distance and near vision with no specific correction for intermediate vision. At virtually every point in the progressive lens, the eye finds the power in the perfect agreement with the distance at which it is focusing (Fig. 11.10).
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Fig. 11.9: No line progressive lens
Fig. 11.10: Progressive lens provides clear vision from far to near
Comfortable Intermediate Vision
The progressive zone of the progressive addition lens gives rise to an area, which provides the clear vision for the intermediate correction. Only in the early stage of presbyopia, can single vision and bifocal wearer enjoy clear intermediate vision, as they can still accommodate and adjust their head position. But for higher additions, progressive addition lens continues to offer clear vision at intermediate distance also. Trifocal lens, despite their clear intermediate field of vision, is not ideal, as the wearer must cope with the image jump at the two segment lenses (Fig. 11.10).
Continuous Support to the Eyes Accommodation
In a single vision-reading lens, the eyes accommodation is supported for near vision only. In a bifocal lens, the eyes accommodation experiences abrupt changes when the gaze shifts from distance to near vision across the segment lines, because the wearer must constantly choose between distance and near vision power and switch from maximum to minimum amplitude of accommodation. For example, consider an eye focusing
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Fig. 11.11: Extreme right picture shows clear intermediate vision through progressive lens
through a bifocal lens at an object moving towards it from a reasonable distance. The eye first uses the distance power, accommodating to its maximum amplitude to focus as the object is drawn within arm’s length. The eye then switches to the near segment where it must totally relax its accommodation for intermediate viewing, before again increasing to its maximum amplitude of accommodation as the object is drawn closer. Thus, the eye varies its amplitude of accommodation twice from minimum to maximum. This adjustment in accommodation would occur only once with a progressive lens, just as it would with natural, non-presbyopic accommodation.
Continuous Perception of Space
Progressive lens also offers continuous perception of space. When an eye looks through a progressive lens, its perception of spatial relationship is uninterrupted. Changes of power are gradual and continuous in all direction with minimum distortion. Single vision reading lenses do not allow real spatial perception. The two portions of bifocal lenses split and alter spatial relationship. Vertical and horizontal lines appear broken and image jump hampers wearer’s vision (Fig. 11.11).
Fig. 11.12: Perception of form and movement through a progressive lens
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PROGRESSIVE ADDITION LENS MARKINGS
All the progressive addition lenses contain important markings, which are used to identify lenses and to assist in their fitting and verification. The important markings are explained below (Fig. 11.13):
Fig. 11.13: Left eye progressive lens
A and A1: They are two hidden circles, which are permanently etched on the lens at 34 mm apart. When the ink marking is removed, they are made visible by fogging.
B:This point is the distance optical centre (DOC) of the lens and is also known as Prism Reference Point.
C:Hidden addition power situated at the temporal side and is made visible by fogging.
D:0-180° axis line passing through the DOC.
E:Fitting cross lies above the DOC.
F:This is the Distance Power (DP) circle to check the exact distance power with the help of lensometer.
G:Hidden logo situated nasally and is made visible by fogging when the ink marking is removed.
H:7mm to 9 mm circle is the centre of the near vision area and is inset by 2.5 mm.
PROGRESSIVE ADDITION LENS OPTICAL DESIGN
Every progressive lens design requires a globally smooth surface that provides a gradual transition in curvature from the distance portion down into the near portion. This gradual blending of curvature means that the addition power is gradually changing across a large area of the lens. Unfortunately, the superior optics and line free nature of progressive addition lens does have a bit of a price to pay, i.e., the change in curvature
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Fig. 11.14: Progressive addition lens design
results in an inevitable consequence in the form of surface astigmatism at the temporal and nasal side. Surface astigmatism produces an unwanted astigmatic error or cylinder error that can blur vision and limit the wearers field of clear vision. Therefore, this astigmatic error essentially serves as a boundary for the various zones on the progressive lens surface. Unwanted cylinder which is the consequence of the lens design is influenced by:
Add Power
The amount of astigmatism will be directly proportional to the add power of the lens. A + 2.00D addition, for example, will generally produce twice as much cylinder error as + 1.00 D addition.
Length of the Progressive Corridor
Shorter corridors produce more rapid power change and higher levels of astigmatism, but reduce the eye movement required to reach the near zone. Larger corridors provide more subtle power changes and lower levels of astigmatism, but increase the eye movement required to reach the near zone of the lens.
Width of Distance and Near Zone
Wider distance and near zones confine the astigmatism to smaller regions of the lens surface, which produces higher levels of astigmatism. However, wider zones do provide larger areas of clear vision. The location of the near zone, which is offset nasally to account for convergence in down gaze, is critical to wearer’s comfort and its position varies between designs.
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To understand how the length of the progressive corridor and the add power can affect the rate of change and magnitude of the astigmatic error, consider a progressive lens with a + 2.50 D addition, which has a 17 mm progressive corridor. This lens will have to change by 2.50 over a distance of 17 mm. This implies plus power represents an average change of roughly 2.50/17 = 0.15 D per millimeter. Therefore, the power of a progressive lens changes more rapidly down the progressive corridor as the addition power increases or the length of the corridor decreases.
A well designed progressive lens will reduce the amount of astigmatic error to its mathematical limits for a given design. During the design and optimization process, various parameters are adjusted to control and manipulate the distribution and magnitude of this astigmatic error across the progressive lens surface. The width of the near and distance zones, and the length of the progressive corridor are the chief parameters that are altered.
The magnitude, distribution and the rate of change (or gradient) of this astigmatism error are all performance factors that can affect the wearers acceptance of the lens. The amount and gradient of peripheral lens aberration determines the field of view, while the gradient and type of aberration primarily influence adaptation time. A steeper gradient concentrates the change in aberrations over a smaller distance and provides wider distance and near zones free of aberration. Astigmatic aberration is generally perceived as distortion or blur while prismatic aberration is perceived as “swim” or “waviness” with head movement.
Fig. 11.15: Power profile for a progressive lens
PROGRESSIVE ADDITION LENS DESIGNS
Basically we can categorize the progressive addition lens design into three groups:
•Mono design and multidesign
•Asymmetry and symmetry design
•Hard and soft design.
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Mono Design and Multidesign
In case of mono design progressive addition lens, a single design is used for all addition powers, i.e. the position for the near vision does not change with the change in near addition power causing difficulties while viewing near objects as the wearer holds reading material closer to him with the increase in his near addition power. Hence, it can never offer optimum comfort to both emerging presbyopes and matured presbyopes, as their needs are different.
In multidesign the position for near vision changes with the addition power change, i.e., the near area goes up with the increase in the addition. All the twelve additions from 0.75D to 3.50D have been studied separately to define the ideal design for each stage of presbyopes.
Asymmetry and Symmetry Design
Fig. 11.16
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In case of symmetrical progressive addition lens design, the right and the left lenses are identical. To achieve the desired inset for the near zone, they are simply rotated by an equal and opposite amount in the two lenses, i.e., 10° anti-clockwise in the right lens and clockwise in the left lens. The principal drawback of this design is the disruption of binocular vision as the wearer gazes laterally across the lens, since the astigmatism differed between the nasal and temporal sides of the distance zone.
Asymmetric progressive addition lens design incorporates a nasal offset of the near zone and has separate design for right and left lens. So, there is no need of lens rotation in this case. This results in same peripheral optical characteristics and better adaptation, improved binocular vision, more visual comfort and better convergence.
Hard and Soft Design
Fig. 11.17
A harder progressive addition lens design concentrates the astigmatic error into smaller areas of the lens surface, thereby expanding the areas of perfectly clear vision at the expense of higher levels of blur and distortion. Consequently, harder progressive addition lens generally exhibits four characteristics when compared to softer designs:
1.Wider distance zones
2.Wider near zones
3.More narrow and shorter progressive corridors
4.More rapidly increasing levels of astigmatic error.
A softer progressive addition lens design spreads the astigmatic error across larger areas of the surface, thereby reducing the overall magnitude of blur at the expense of narrowing the zones of perfectly clear vision. The astigmatic error may even encroach well into the distance zone. Consequently softer progressive addition lens generally exhibits four characteristics when compared to harder designs:
1.Narrower distance zones
2.Narrower near zones
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3.Longer and wider progressive corridors
4.More slowly increasing levels of astigmatic error.
In general, harder progressive addition lens designs will provide wider fields of view, and will require less head and eye movement, at the expense of more swim and blur. Softer progressive addition lens designs provide reduced levels of astigmatism and swim while limiting the size of the zones of clear vision and requiring more head and eye movement.
However, modern progressive addition lenses are seldom absolutely “hard” or absolutely “soft”. Unfortunately such terms do not accurately describe modern lenses. Many of the recent progressive addition lens design incorporates the best balance of these two design philosophies to show following characteristics:
1.Larger effective distance and near zones
2.Peripheral aberrations are well controlled to enable the wearer to adapt easily.
3.Combination of hard and soft design.
Recently, SOLA optical introduced another concept i.e. DESIGN BY PRESCRIPTION the use of different distance designs for different distance refractive errors. For example, the design employed for the 7.25 base curves varies slightly from the design employed for the 5.25 base.
OPTICAL DESCRIPTION OF PROGRESSIVE ADDITION LENS
Manufacturers have made various attempts to represent the size and location of the optical zones and peripheral aberrations of the progressive lens. Graphically, there are four common ways of progressive addition lens design representation:
Power Progressive Profile
The primary function of a progressive addition lens is to restore near and intermediate vision while maintaining clear distance vision. While distance power and the near vision power in any given lens are fixed parameters, the lens design must define the progression of power change from one to the other. Physiological considerations favor a high location of the near vision zone; however, a short abrupt power progression from distance zone to near zone will usually create the rapidly varying aberrations in the lens periphery that cause discomfort. The great challenge for the designer is to manage both the length of the power progression and the rate of power change in order to create comfortable exploration of distance, intermediate and near visual fields without excessive, and tiring vertical head movements and ocular effort. The rate at which the power increases over the progressive zone is governed by the power law for the design. The power law may be
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linear as assumed in Figure 11.18 or it may be more complex to provide a greater or less increase in power at the start of the progression as assumed in the Figure 11.19.
The curve so drawn represents the power progression of the lens along its meridional line from distance to near vision. The power progression is the result of a continuous shortening of the radius of curvature of the front surface.
Fig. 11.18: Power profile for a progressive lens with a linear power law
Fig. 11.19: Comparison of isocylinder lines for hard and soft design of power, plano add + 2.00D
Contour Plots
The contour plot is the most common method used to represent progressive addition lens designs. It describes the surface power of the front surface of the lens and identifies zones of constant surface power indicated by contour lines. Contour Plots also attempt to show the size of the distance, intermediate and near zones as well as the extent and gradient of peripheral aberrations.
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There are two types of contour plots:
•Isosphere contour plots
•Isocylinder contour plots
Isosphere Contour Plots
This is a two dimensional map of the lens representing the distribution of spherical power across the lens. This form of graphical representation divides the lens along lines of equal dioptric values. Each contour line/ shade represents an increasing level of power at given interval. The value of this line/shade is chosen arbitrarily, usually at the increments of 0.25D or 0.50D to 1.00D. Between two consecutive lines, sphere value varies by a relatively constant rate. Figure 11.20 shows isosphere contour plot of a progressive addition lens. In addition to describing sphere distribution the lens designer may communicate information about the location and size of the near vision zone using an isosphere contour plot.
Fig. 11.20: Power contour plot of a PAL (+ 2.00 with a addition + 2.50D)
Isocylinder Contour Plots
Isocylinder contour plots are two-dimensional map that divides the lens design into ranges of cylinder levels through out the lens. Most of the time isocylinder plot is misused by the lens designer in the promotion of their progressive addition lenses. It is routinely misapplied by some lens marketer seeking to establish competitive claims such as “a larger reading zone” or “less unwanted astigmatism”. Since induced cylinder is disturbing but
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Fig. 11.21: Astigmatic contour plot of a PAL (+ 2.50 add)
unavoidable by progressive addition lens design process, it is legitimate for the lens designers to map the location and degree of cylinder with each design interaction. “Soft” design have fewer zones of induced cylinder, represented by fewer lines on the isocylinder contour plots, located further apart than in “hard” design. Hard designs, on the other hand, have a greater number of lines located closer together. To the untrained observer, the hard design may appear superior because of the longer areas free of induced cylinder. However, clinical experience has shown that wearers perceive soft design as more comfortable in dynamic and peripheral vision as they do not have a high concentration of cylinder in areas critical to peripheral and dynamic vision, as in the case with hard designs.
Lens designers also use isocylinder contour plots to depict the presence or absence of asymmetry in the progressive addition lens design by comparing the nasal and temporal aspects on either side of the oblique path of power progression. An asymmetric design produces identical optical characteristics on both sides and thus, sphere power cylinder, and vertical prism are almost identical for both eyes in any direction of gaze, promoting binocularity and comfort. A symmetric design employs a single design that is rotated about 10 degrees in one direction to create a “right” lens and about 10 degrees in the opposite direction to create a left lens. In a hybrid of these two approaches, referred to as dissymmetric design, separate designs are used for right and left lenses. However, since the entire unwanted cylinder has been pushed from the temporal to the nasal side, different optical characteristics are present on either side of the lens.
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Grid Plot (Fig. 11.22)
The grid highlights the distribution of the prismatic effects of the lens by showing how they alter a regular rectangular grid.
Fig. 11.22: Grid plot a PAL (+ 2.50D add)
Rate of Change Plot (Fig. 11.23)
Figs 11.23A to C: (A) 3D power plot of a PAL (+ 2.50 ADD), (B) 3D astigmatism plot of PAL (+ 2.50 ADD), (C) 3D power gradients plot of PAL (2.50 ADD)
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Rate of change plots are the three dimensional graphical representation of the variation in the optical characteristics of progressive addition lenses, which plots vertically the value of a given optical characteristics at each point of the lens in relation to a reference plane. It may be used to show:
a.Distribution of power
b.Astigmatism
c.Gradients of power variation
Rate of change plot is a better way to visualize the softness or hardness of a lens design. They represent the rate of change in the sphere value between two given points on the lens surface. The lower the difference in the sphere power between two points, the lower the plotted altitudes. Thus, threedimensional plots are more demonstrative of lens characteristics than contour plots. Flatter plots indicate softer design. The lens designer can manage the softness in one of the two ways.
1.By lengthening the progression. Or
2.By carefully controlling the rate of change of the optical characteristics between all points on the lens surface.
Since the length of the power progression is not easily demonstrated on the rate of change plots, they give incomplete picture of the overall lens design. Otherwise this is a more demonstrative procedure.
The graphical representations are useful tools to communicate the geometrical features of PAL design to a trained observer. However, they do not really correlate with wearer’s acceptance, as this is dependent on the sum total of many different factors - astigmatic error, the power rise, prismatic effects, distortions, the required dioptric power, cent ration, the frame fit, vertex distance and the subjective impression of the wearer. So the success of a progressive addition lens design should be judged on extensive wearing trials rather than studying as these plots.
DESIGNING PROGRESSIVE ADDITION LENS
A progressive addition lens is designed not only to provide a presbyope – an ability to see clearly at all distances but also to respect all the physiological visual functions like foveal vision, extra foveal vision, binocular vision, etc. The foveal area of the retina permits sharp vision at any distance within a small field which follows the eyes rotation which is usually within 30° angle.
Within this end the lens areas used for foveal vision must provide for perfect retinal images. The wearers natural body and head positions determine the vertical rotation of the eye for near and distance vision, and therefore the optimal length of the lens power progression. The coordination of the body, head and eye movements in relation to the objects
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location in the vision field defines the power value needed at each point of the progression. The horizontal eye and head movements determine the field of gaze and defines the width of the lens zone used for foveal vision. And to maximize wearers visual acuity in the lens central area, the unwanted induced cylinder of the progressive lens must be kept to a minimum and be pushed to the periphery of the lens.
Although, extra-foveal vision do not provide sharp vision but the wearer do locate the object in space, perceive their forms and detect their movements through extra-foveal vision. The prismatic distribution on the progressive addition lens surface and its magnitude introduces slight deformation of horizontal and vertical lines, thus altering the wearers visual comfort. The whole of the retina is almost homogeneously sensitive to motion. The variation of prismatic effects plays a role in the wearer’s comfort where it must be slow and smooth across the whole lens to ensure comfortable dynamic vision (Fig. 11.24).
Fig. 11.24: Horizontal eye/ head movement coordination and width of field
Binocular vision refers to the simultaneous perception of the two eyes (Fig. 11.25). For perfect fusion, the image produced by the right and left lenses must be formed as corresponding retinal points and display similar optical properties. The eyes naturally converge when the wearers gaze is lowered for near vision. The power progression must be positioned in the lens in such a manner that it follows the eyes path of convergence downwards in the nasal direction. Both right and left lenses must offer approximately equal vertical prism on each side of the progression path. The retinal images formed in both eyes must be similar in all directions of gaze. For that purpose, the power and astigmatism encountered on corresponding points of right and left lenses must be approximately equal.
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Fig. 11.25: Binocular vision with progressive lenses
In designing the following zones of the progressive addition lens, the lens designer works towards respecting these physiological functions:
Vertical Location of the Near Vision Area
Higher position of near vision area relieves strain to extra ocular muscles and eases binocular function with downward gaze. But a shorter progression usually results in rapidly varying peripheral aberrations. A good compromise consists in locating the usable near vision at a downward gaze position of about 25°.
Fig. 11.26: Progression of power in relation to head posture and downward eye movement
Power Progression Profile
A suitable power progression along the meridional line enables the wearer to explore the object field without tiresome vertical head movements. This is achieved by associating the shape of the power progression to the
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orientation of the vertical horopter linked to the natural tilting of reading material (Fig. 11.26).
Horizontal (lateral) Location of the Near Vision Area
Once the power profile has been defined, its lateral positioning on the lens must be adapted to the natural convergence of the eyes and the value of the addition. With the advanced presbyopia, the reading distance becomes closer, the meridional line, therefore, must be shifted nasally as the addition increases.
Besides, balancing of vertical prism between right and left lens is very important to respect retinal image fusion in binocular vision. This is achieved by an asymmetrical design of the progressive addition lens surface coupled with proper positioning of the meridional line. In the lens periphery, image quality constraints are less demanding, while the control of prismatic effects is of utmost importance for motion perception.
All of the above optical requirements are introduced in the Merit function and are then integrated into the lens design optimization software. The merit function evaluates the numerous points of the lens and also overall performance of the lens by the weighted sum of the found merit function values and then the lens design is optimized. Numerous lens prototypes of each design are then produced and tested through rigorous clinical trials. Comparative lens evaluations are made after in depth analysis and patients comments, leading to a final selection of the progressive addition lens design.
PRISM THINNING
The progressive addition lenses are designed by increasing the curvature of the progressive zone in the near vision area. As a result of this, the progressive addition lens is thinner at the bottom and thicker at the top. To
Fig. 11.27: Thinning prism
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produce a thinner lens, the lens surfaces generally use an “equithin” technique which consists of incorporating a vertical prism in order to reduce the thickness and weight of the lens in the back surface specification of the lens. Since the same amount of vertical prism is incorporated into both right and left lenses, there is relatively no prismatic effect for the wearer. This prism is often referred to as Yoked prism, as no net binocular prismatic effect is produced.
So prism thinning is the process of grinding prism into a progressive lens blank to reduce the thickness difference between the upper and the lower edges. Prism thinning typically involves grinding base down prism into the progressive lens. In addition to balancing the thickness difference between the top and the bottom of the lens blank, prism thinning also reduces the centre thickness of the lens with plus and/or higher add powers. The overall reduction in thickness also makes the lens lighter in weight.
Prism thinning is accomplished during the generating process by literally tilting the front surface of the lens on the chuck of conventional generator, using a base up positioned prism ring. When the back surface is ground normally, the surfaced lens is left with a prismatic effect at the centre. Newer three axis generators produce this prismatic effect without the use of prism rings grinding the back curve with a tilt. The end result in either case is the reduction of unwanted thickness.
Fig. 11.28: Base down prism is ground into the lens blank using prism-ring (with its base positioned up) with conventional generators
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The amount of prism thinning used is roughly equal to the 2/3rd of the addition power. This is often recommended when the power through the vertical meridian of the lens exceeds + 1.50 D or so. This formula does not consider factors like the fitting height and the distance power, but still produces satisfactory results in most cases. It is possible to prism thin the minus powered lenses as well. Depending upon the fitting height either base down or base up prism may be required to balance the thickness difference.
We can verify this prism onto the progressive addition lenses by placing the prism reference point of the lens in front of the centre of the lens of the focimeter. This is very important especially where only one lens is being replaced. If the previous lens had not been prism thinned and the new one was or vice-versa – an unwanted vertical prism imbalance will be induced. In summary prism thinning is a useful tool that improves both the finished cosmetics of many progressive addition lenses with little visual impact to the wearer.
LIMITATION OF CONVENTIONAL PROGRESSIVE ADDITION LENSES
With all the advancements from hard to soft design and from a required minimum fitting height of 26 mm to 14 mm, many eye care professionals assumed that progressive addition lenses had come relatively close to perfection. However, this is not true. There are still four basic limitations in conventional progressive addition lenses created by their inherent designing:
Fig. 11.29: Progressive addition lens magnifies the image in its various section
1. Differing Magnification throughout the Lens
The changing curves on the front lens surface and the change in power through out the channel and reading portion of the lens create varying magnification through out the lens. The magnification increases throughout the progressive zone. The result is that the vertical lines viewed through the progression zone exhibit skew distortion.
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Fig. 11.30: Skewed distortion in a progressive lens
2. Restricted Visual Field
Progressive addition lens restricts our field of vision particularly for intermediate or near areas. The curves are always on the front side of the conventional progressive addition lenses which position the visual area of the lens at considerably away from the eye. The ultimate result is restriction of visual field.
3. Compromised Optics
Conventional progressive addition lenses are produced in varying base curves averaged for a wide range of prescription and a nearest suitable front base curve is used for a given prescription. Ideally the base curve selected should match the exact curves used on the backside. This is simply not possible in conventional progressive addition lenses. As a result acuity is compromised to some extent in all visual areas, depending on how close the front base curve is averaged for the patients prescription.
4. No Control over the Inset of the Near Portion
Some progressive addition lens designs do have an inset, which varies with addition and/or distance correction, but not yet with interpupillary distance.
5. Difficult to check with Focimeters
Progressive addition lenses are relatively difficult to check with the help of focimeter, specially if the original engravings including those indicating distance power circle and near power zone cannot be found.
REMARKING PROGRESSIVE ADDITION LENSES
Majority of the progressive addition lens manufacturers employ laser etched molds or fluorescent markings on the front lens surface to help dispenser
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Fig. 11.31
identify the lens brands, lens style, addition power, distance power circle and near power circle and thereby enabling the dispenser to fit the lens into the frame properly. These micro engraving, if removed can be detected under a bright light a using a dark background.
Some manufacturers also make instruments with appropriate lighting background and magnifier to make viewing the markings easier.
Fig. 11.32: Essilor’s PAL ID
The first step in identifying a progressive addition lens is to locate the hidden circle or manufacture’s symbol located on the 0-180 degree line, 4mm below the fitting cross on both nasal and temporal sides of the lens. Below these two markings, we find the near addition power (sometimes abbreviated) on the temporal side and the symbol identifying the particular lens on the nasal side. Once the two hidden circles and the brands are
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identified, place the lens on the respective brand layout card with two circles of the lens coinciding with the two circles on the card. Now mark the other markings.
The following options may be followed:
The microengraving can be detected by reflecting light from the overhead lights off the lens surface.
Fig. 11.33: Stripped grid
Or, fogging the lens surface by breathing warm moist air onto the lens surface.
Or, position the light source behind the lens.
Or, hold the progressive addition lens 10-15 cm in front of the striped grid. Focus on the lens surface.
The marking will stand out against the background as shown in the Figure 11.34:
Fig. 11.34: Hidden circles relocated
GENERATING PROGRESSIVE POWER SURFACE
A progressive power lens surface is an aspheric surface of the nonrotationally symmetric type, generated by means of computer numerically controlled (CNC) machining of either the surface itself or the mould from
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which the surface may be cast or slumped. The actual geometry of a given progressive lens surface is always regarded as proprietary information of the lens manufacturers. However, some insight into how the design of a surface might proceed can be obtained by the following illustrations:
To understand the geometry of progressive addition lens surface, we need to consider the E-style bifocal with two different spherical surfaces placed together so that their poles share a common tangent at point D, as shown in Figure 11. 35, where two surfaces are continuous.
At all other points, there is a step between two surfaces, which increases with the increase in distance from the point D. To produce a truly invisible bifocal design, the two surfaces must be blended together such that the DP surface and NP surface are continuous at all point.
A progressive lens may be considered to have a spherical DP and NP surfaces connected by a surface that’s tangential and sagittal radii of curvature decrease according to a specific power law between the distance and near zones of the lens. Theoretically, to make a surface with curvature that increases at the correct rate to satisfy the given power law, we need to combine small segments of spheres of ever decreasing radii, all-tangential to one another in a continuous curve. These sections will be continuous only along a single so called meridian or umbilical line and at all other points of the sections; the surface of the sections must be blended to form a smooth surface.
Fig. 11.35: E-style bifocal made by placing together two spherical surfaces with a common tangent at D
The simplest concept of this can be explained with a section taken from an oblate ellipsoid, as shown in the Figure 11. 36, where the radii of curvature of the spherical surfaces which represent the distance and near portions are shown as rD and rN respectively. It can be seen that the solid ovoid, which is obtained by inserting the ellipsoidal section between the two hemispheres shown in the figure, will result in a surface which has no
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Fig. 11.36: Concept of a progressive lens surface. A section of an oblate ellipsoid is inserted between two hemispheres of radius of curvature rD for the distance portion and rN for the near portion
discontinuities. Along the meridian line, DD1, a cross section through the surface would be circular and the radii of the circles in a plane parallel with either the distance or near portion circles do indeed decrease from rD to rN continuously.
The ability to cut surfaces of such a complex nature was made possible by the use of computer numerically controlled (CNC) grinding machines. The single point diamond tool cuts the surface under computer control, sweeping in an arc over the work piece with the program positioning the cutter in exactly the right place as the cutting tool traverses the work piece.
The work piece could be a glass blank or a glass mould from which plastic lenses might eventually be cast, or it could be a ceramic block upon which glass blanks or finished lens could be slumped with or without vacuum assistance.
The important stages in the production of progressive power surface are as under:
1.Surface designing and numeric description.
2.Generating the aspheric curve.
3.Grinding, smoothing and polishing
4.Inspection.
Surface designing and computation of the surface topography is
translated into numeric data in the form of co-ordinates that can be fed directly into the CNC generators. As much as 50000 numeric data point may be required for an individual progressive power surface. The basic reference for the progressive power surface is usually taken to be the
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spherical surface of curvature required for DP curve. Then the spherical curve is subsequently made aspheric in accordance with the design philosophy for the lens. Before that the spherical convex surface must be smoothed and polished accurately so that its topography has precisely the known desired characteristics.
CNC diamond tool is used to create the aspheric surface as explained earlier. If the work piece is glass blank, it must be smoothed and polished, using floating pad system. It is essential to ensure that the pads do not remove any more glass material than intended; otherwise an accurate surface geometry will not be maintained.
Ceramic slumping moulds produced by CNC cutters can be used to produce progressive power surface. The glass blanks are placed on ceramic moulds as shown in the Figure 11. 37. The entire assembly is then heated to the high temperature at which the glass starts to flow.
The glass slumps and the geometry of the convex surface of the mould is transferred to convex surface of the blank. The entire process has to be done under highly sophisticated temperature control to ensure that the glass flows correctly. Slumping can be assisted by vacuum forming also, whereby the glass is heated to just beyond its softening point, whereupon, a vacuum is applied to the interface between the forming block and the concave surface of the blank – which pulls the surface into shape.
It is the dream of the lens designer to create a progressive addition lens with no areas of distortions at the either side of the lens, which unfortunately is just not possible. The distortion on the either side is the result of induced cylinder, which is due to the presence of unwanted surface astigmatism and is evident during the lateral gaze within the progressive zone. All that the designer can do is to reduce distortions to the minimum. Progressive designs vary from one to another only by how these “no go” areas of the lens are distributed across its surface.
Fig. 11.37: Slumping a glass blank to a required geometry
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The astigmatism across the pupil is inversely proportional to the length of the progression zone. The longer is the progression zone, the smaller is the surface astigmatism. In other words the rate of change in the surface astigmatism in the periphery depends upon the addition. Higher addition gives faster changes and hence peripheral astigmatism is more evident.
The reduction of peripheral distortion is also accomplished by using the conic sections of changing asphericity rather than spherical sections as power is increased in the vertical meridian. These varying sections reach a relatively uniform power at a given peripheral portion of the lenses so that prismatic and cylindrical effect at these edges are much the same through out the peripheral vertical range of the lens. To achieve this in the upper portion, the distance section is increased slightly in power towards the periphery by + 0.30 D. This type of design also reduces the “optically pure” zone for near in width by allowing the reduced remaining power variation to spread over more area. As the curves are aspherical, the near zone reveals a spherical area of only 12 mm wide. However, the power change is so gradual that a much wider defective zone of practically equivalent power exists, depending upon the power of addition.
EVOLUTION OF PROGRESSIVE ADDITION LENS
The first commercially successful progressive addition lens was introduced by Essel under the name of Varilux in 1959. The design consisted of large spherical distance and near zones, linked by a series of circles of ever decreasing radii between the distance vision sphere and the near vision sphere. More attention had been given to ensure larger distance and near zones than to the quality of peripheral vision on either side of the progressive corridor as shown in Figure 11. 38. Today such a design would be described as very hard design.
The second generation of progressive lenses, Varilux 2 was introduced in 1973. It too provided large distance, intermediate and near fields of vision, but still it was at the cost of quality vision in the lateral regions of the progressive zone. Conic sections of changing eccentricities from one section of the lens to another replaced the circular section employed in Varilux 1 design. The effect of this was the reduction of power in the periphery of the progressive zone. The new concept of “horizontal optical modulation” was introduced which took care of the extra-foveal vision also. Binocular vision was optimized as a result of an asymmetrical design. The overall design of the lens is represented by a succession of conic sections as shown in the Figure 11. 39.
During the next decade other manufacturer introduced progressive addition lens design focusing specific optical characteristics. Some emphasized on large distance and near vision zones, concentrating the
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Fig. 11.38: First progressive addition lens
Fig. 11.39: The “Physiological” progressive addition lens
inevitable astigmatism in the lens periphery. American Optical Ultravue, Rodenstock Progressive R, Sola Graduate etc., were in this category. Other manufacturer took a different approach, reducing the amount of unwanted astigmatism in the periphery by spreading it more widely in the lens. Truvision Omni of American Optical became popular. Zeiss Gradal HS placed special emphasis on the concept of the lens asymmetry and comfortable binocular vision.
Soon multi-design concept was introduced in Varilux Infinity in 1988
– the third generation progressive. The multi-design used district designs to match the wearers changing needs with advancing presbyopia. Multidesign concept refers to the change of power progression profile with
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near addition, generally resulting in a larger progression length for a low addition and hence soft design. The corridor length becomes shorter and the design harder as the addition increases. The multi design concept is well illustrated with the change of power progression profile by addition in the Figure 11.40.
Essilor introduced the fourth generation of progressive lenses under the brand name of Varilux Comfort in 1993, which offered more natural vision than any previous progressive lens designs. The near vision area in this lens design is located high, so that the wearer can reach it easily and naturally when lowering their gaze. Fewer head and eye movement are required to explore the near and intermediate vision fields and hence the wearer enjoys more comfortable posture. These advantages were because of specific power profile in the progression zone adopted for Varilux Comfort. For example in case of + 2.00 D addition, 85% of the full addition is reached just 12 mm below the fitting cross compared to a minimum of 14mm or 15 mm for previous progressive designs. Due to the softness in the lens periphery, the wearer enjoys additional comfort in the peripheral and dynamic vision as the necessary horizontal head movement required to explore the full of the field was greatly reduced. The asymmetry design incorporated also offered balanced binocular vision, while at the same time integrating the multi design concept of previous progressive lens design. Several competitive designs from other manufacturer appeared during the decade.
In 2002, the fifth generation progressive design – “Varilux Panamic” was introduced. This new design was based on the success achieved by Varilux Comfort in providing a larger field of vision, which resulted from the softness and regularity of the lens periphery. The Varilux Comfort
Fig. 11.40: Multi-design progressive addition lens
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design showed that the vision was a global process and the wearer would perceive their field as larger if they had the ability to see comfortably through the lens periphery. In the Varilux Panamic design, the balance between the control zone and the periphery was shifted towards the peripheral zone of the lens, so that peripheral effect were lens dramatic than the previous designs. This change also brought the advantages of reducing the swimming effects obtained by rotation of the head and eyes around the field and improving binocular fusion in the lens periphery. Clinical trials have found significant enlargement in the field of vision in all zones of the lens.
New generation designs from other manufacturers include the BBGR Evolis, the Nikon Presio W, the Pentex Super Atoric F, the Seiko P-ISY – the first of the new designs to have the progression on the concave surface, Hoya’s Hoyalux D design, is double sided progressive.
The most recent development in the progressive addition lenses is the Varilux Ipseo from Essilor – the personalized progressive lens that takes into account the actual degree of head and eye movement which the wearer employs when viewing through the intermediate and near zones of the lens. Research has been shown that each individual has specific head and eye behavior, which can be measured and then the progressive surface is designed to incorporate the wearers behavior.
Over the last few years rapid strides have taken place in ophthalmic lens production technology. Through a combination of better software, faster computer processing and free form surfacing, it has become possible to modify some aspects a progressive design to take into account the individual characteristic of the frame in which the lenses are to be mounted.
CURRENT PROGRESSIVE LENS DESIGN DEVELOPMENT
With the changing scenario in the trend of spectacle dispensing, lot of new development have come in progressive addition lens designing, with special regard to the followings:
Shorter Corridor Progressive Lens
Fig. 11.41: A typical shorter corridor progressive
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When the progressive lenses were, first introduced, the recommended minimum fitting height was 22 mm. That was fine when the eye sizes of the spectacle frames were large and had deep ‘B’ measurements. However, as the smaller and narrower frame shapes become popular, a need for progressive with shorter corridor was crucial to their continued success. Smaller frames proportions differ from large frames in that they have a higher horizontal to vertical ratio. In other words the lens has to be not only shorter vertically but also relatively wider. The second challenge is to provide distortion free distance vision in the lens periphery. Finally the shorter corridor lens must account for the fact that the wearer’s head and eye movement are influenced by frame size. Wearer of small frames tend to move their head more and eyes less when transitioning from distance to near viewing than that of large frame wearers. When looking down a short corridor lens, the eyes simply run out of the lens, so the head has to move to maintain focus on the items of regard.
To make the progressive addition lens work in small frames, many lens manufacturers came out with shorter corridor progressives. In 1999, American Optical introduced A.O. Compact – the first short corridor progressive lens. A.O. Compact has a 13 mm corridor that permits full reading function with a minimum fitting height of 17 mm. Soon other companies followed the suit and shorter corridor progressive becomes popular: Nikon Presio, Essilor Ellipse, Shamir Piccolo, Hoya Hoyalux Summit CD, Kodak Concise, Pentex Mini AF etc., became popular.
Internal Progressive
Internal progressive addition lens is a big step forward in overcoming many of the limitations of progressive lenses widely available today. Seiko is proud to have designed and patented the world’s first internal progressive lenses. In an internal progressive lens, the curves producing changing power are positioned on the back surface of the lens, and front surface is spherical. The backside is a free form surface with progressive surface and cylinder surface on the back. Therefore, it is possible to have a spherical front curve. The design provides a field of vision that is 30% wider than a front side progressive lens. This is due to the fact that the progressive surface rests closer to the eyes. Besides Seiko, Rodenstock ILT is also being test marketed in the USA. The new internal design offers following additional advantages:
1.Expansion of the visual fields
2.Magnification differences between various areas of the lens are reduced.
Customized Progressives
Customized or personalized or individualized progressive surface design is the most unique and the latest development in the progressive addition
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lenses. The latest design from Essilor’s Ipseo takes into account the actual degree of head and eye rotation that the wearer employs when viewing through the intermediate and near zones of the lens. Research has shown that each individual has specific head and eye movement behavior, which can be measured and the progressive surface can be designed to incorporate the wearer’s behavior. This principle has been incorporated into the personalized progressive design from Essilor-the Varilux Ipseo.
Fig. 11.42: Vision print system
A new instrument the vision print system as shown in the Figure 11.42, has been developed to determine eye/head movement for a specific individual, the result of which can be incorporated into the progressive surface. The system consists of three lamps – the central lamp is viewed at 40 cms from the centre of the wearers forehead, with two separate lamps, 40 cms on either side of the central lamp. The wearer is directed to look at the lamp, which is illuminated and their head movement is recorded by an ultrasonic signal that is emitted by the system and reflected by a transponder attached to the special trial frame the wearer is wearing. As far as the wearer is concerned, the lamps are illuminated at random following a short 15 second cycle and it continuous over a 90 second cycle which enables the systems to calculate and display the eye/head movement ratio, along with a consistency factor referred to as the “Stability co-efficient” (ST). ST is used to adjust the rate of transition between the central and peripheral zones of the lens. Most people’s eye/head ratio is relatively stable, having stability co-efficient below 0.15. ST value 0.00 shows the high stability of head and eyes movement. The indicator A in the instrument indicates head/ eye proportion, the number at B shows the head/eye movement ratio and the number at point C shows the stability co-efficient. The number 0.31 in the Figure reflects that the subject is performing 31% of the gaze movement with their head and 69% with their eyes. The ST of 0.07 indicates high stability of the head and eye movement behavior.
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Fig. 11.43: The information displayed on the screen
Fig. 11.44: Studies show that the head-eye movement ratio directly influences the area of the lens that is used by the wearer
The eye mover uses more of the surface of the lens, so they need a larger acuity zone. For them the harder design as shown in the Figure 11. 45 can be more useful. On the other hand the “head-mover”, having more dynamic vision, will be more influenced by the periphery of the lens as their head moves in their visual world and will be benefited from the peripheral “softness” as shown in the Figure 11. 45. Each Varilux Ipseo lens is designed
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Fig. 11.45: Progressive designs for eye turners and head turners
for the individual wearer, manufactured by free-form technology and to recognize its individual nature, micro-engraved with the wearers initials.
WHO IS SUITABLE FOR PROGRESSIVE ADDITION LENSES?
(Criteria for good candidate)
The introduction of numerous varieties of progressive lenses has made it suitable for almost every presbyopes, for every visual demand and for every wallets, Still proper selection of an ideal candidate is the first criteria for the successful dispensing of progressive addition lens. Before identifying a suitable candidate, the evaluation must be done on the following factors:
General Factors
Careful consideration is needed before advising progressive addition lenses for those with any history of difficulties in adapting to changes in lens power or frame styles. Persons having an interest in trying new things can be good candidates. People with small pupil are more suitable for progressive addition lenses as the effect of peripheral aberration is reduced. An unsatisfied patient with other designs of bifocals may found himself motivated to adjust with progressive lenses. The patient with narrow papillary distance and wider facial width may also be an unsatisfied user of progressive lenses.
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Visual Needs
Information about the patients visual needs enables the practitioner to understand whether the progressive addition lenses will meet the patient expectations or not. Those who require a larger field of view can immediately be eliminated from the consideration. An avid user of computers will be benefited more by occupational lenses rather than a progressive addition lenses. The progressive addition lenses for him may be prescribed as a supplement for general use. Most commonly used near working distance also influence the selection criteria. A patient who invariably works at shorter and middle distance will be a happy user of progressive addition lenses, whereas a constant user of shorter reading, working distance will prefer to have bifocal lenses. Relative use for distance/ near visual zones also sometimes influences the criteria. Myopes who depend more on distance visual zone have always been more successful user of progressive lenses.
Refractive Status
Progressive addition lenses should be avoided for that refraction which may induce vertical prism between the eyes including different refraction between the eyes by more than 2.00 D in spherical power or more than 2.00D in effective cylinder power especially in the vertical meridian (horizontal axis).
IDEAL FRAME SELECTION FOR PROGRESSIVE ADDITION LENS
The ideal frame selection is an important criterion for the successful dispensing of progressive addition lenses. Many a times it has been seen that in spite of having perfect refraction and fitting, the patient is not the happy user of the progressive lenses and the reason established is not other than poor frame selection. The frame should be selected in such a manner so as to cover adequate facial width and also minimizes the vertex distance. Metal frames with adjustment nose pad are more suitable than plastic frames as the fitting height can be adjusted, if required, using the nose pads. While selecting the suitable frame for an individual who is opting for progressive addition lens, the following two factors must be taken into consideration -
Shape of the Frames
Square and panto shapes as shown in the Figure are the most suitable shapes for the progressive addition lenses. Steep rectangular shapes extending beyond the temporal bone of the face may not be suitable. Another important factor in frame selection is ensuring that the shape of the frame
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Fig. 11.46: Ideal square shape of frame for progressive addition lens
Fig. 11.47: Ideal panto shape of frame for progressive addition lens
Fig. 11.48: Aviator shape cuts away the reading area in the nasal side
does not reduce the size of the reading area by being too “cut away” in the nasal area of the frame. For example, an aviator shape frame coupled with narrow pupillary distance may present such a difficulty.
Size/depth of the Frame
The frame must have sufficient depth to accommodate the entire zones of the progressive lens from the top of the distance power circle to the bottom of the near power circle within its aperture, i.e., the ‘B’ measurement should be adequate to include full distance and near optical zones. Another
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important factor, which is usually overlooked, is the ‘A’-measurement of the frame, i.e., it must minimize the distance between the position of the fitting cross and the temporal end of the frame.
Fig. 11.49: Sufficient height between the pupil and the lower rim of the frame
Once the lenses are properly marked and verified on the wearer’s face, the layout card can be used to ensure the confirmation of above factors.
DISPENSING PROGRESSIVE ADDITION LENSES
Progressive lenses should be centered with respect to the eye so that the vision zone can be used ideally at all distance. The fitting of progressive lenses should be carried out with normal, relaxed head and body position and with the eyes looking horizontally straight ahead. The fitting can be done using manual measurement. But before that it is important that you fit the selected frame on the wearers face and then take the measurement. The following adjustment needs to be confirmed:
Frame Front Adjustment
Adjust the angle of the temples so that the frame sits squarely on the face.
Fig. 11.50: Front of the frame not in alignment
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Pantoscopic Tilt
Adjust temples to achieve 8 to 12 degrees tilt, avoid contact with cheeks.
Fig. 11.51: 8 to 10 degree Pantoscopic tilt
Facial Wrap
Ensure that the front of the frame follows the line of the face but not excessively.
Fig. 11.52: Poor facial wrap
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Temple Length
Adjust length of temple to minimize sliding.
Fig. 11.53: Needs to lengthen side bend
Vertex Distance
Minimize Vertex distance by adjusting nose pads but avoid contacting the eyelashes with the lens.
Once the above adjustment are done, take the following two measurement:
1.Monocular PD for distance
2.The fitting height
Monocular PD for Distance
The monocular PD is the distance from the centre of the nose bridge to the centre of the pupil, specified to the nearest 0.5mm. A number of devices can be used to measure monocular PD:
1. Pupillometer
Fig. 11.54: Measuring PD with Pupillometer
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2. PD Ruler
Fig. 11.55: Measuring monocular PD with ruler
3. Marking directly on the lens insert
Fig. 11.56: Marking on the lens inserts
After determining the monocular PD, mark their location on the lens insert in the following ways:
•Place the frame symmetrically and level on the progressive addition lens layout card.
•At the determined PD, mark a vertical line on each lens insert.
•Place the frame back on the wearers face to check the accuracy of PD.
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Fig. 11.57: Marking vertical lines on the layout
Fitting Height
The fitting cross on progressive addition lens must coincide with the pupil centre of the wearers in their natural posture. The position of the pupil centre relative to the correctly adjusted frame is the fitting height, and is specified as the distance above the deepest point of the inner frame rim to the nearest 1mm. Occasionally some practitioners prefer to specify the fitting height as mm above datum, which is the line halfway between the uppermost and lowermost rims of the frame.
The following procedure is used to measure the fitting height:
•Place your self opposite and at the same height as the wearer.
•Ask the wearer to adopt a comfortable posture and look straight ahead.
•Ask the wearer to look at your LE.
•Hold a pen torch just below your LE.
Fig. 11.58: Fitting height measurement
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•Close your RE to avoid parallax error.
•Observe the position of the light reflection in the wearers RE relative to the vertical PD line already marked on the lens.
•Place a small horizontal mark on the PD line corresponding to the pupil centre.
Fig. 11.59: Fitting height measurement (29 mm in this picture)
•Ask the wearer to look at your RE and complete the procedure for the other eye.
•Move the frame up and down slightly, let it settle, and report for both eyes.
•Confirm the final marked lens insert.
The ideal position of the fitting cross should be as shown in the Figure 11.60:
Fig. 11.60: Ideal position of the fitting cross